Studies On The Treatment Of Coal And Brewery Wastewater Using Adsorption And Coagulation– Flocculation Techniques

2.7.3 ADSORPTION ISOTHERMS
The process of Adsorption is usually studies through graphs known as adsorption isotherm. It is the graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature. (Xamplified.Com/adsorption,2010).

2.7.3.1 TYPES OF ADSORPTION ISOTHERMS.
a).Freundlich Adsorption Isotherm: In 1909, Freundlich gave an empirical expression of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as Freundlich Adsorption Isotherm or Freundlich Adsorption equation or simply Freundlich Isotherm.

x /m = KP

where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p and k, n are constants whose values depend upon adsorbent and gas at particular temperature. Though Freundlich Isotherm correctly established the relationship of adsorption with pressure at lower values, it failed to predict value of adsorption at higher pressure (www. Xamplified. Com/adsorption,2010).

b). Langmuir Adsorption Isotherm:
In 1916, langmuir proposed another Adsorption Isotherm known as langmuir Adsorption isotherm. This isotherm was based on different assumptions one of which is that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules.
A (g) + B (s) Adsorption AB
desorption
Where A (g) is unadsorbed gaseous molecule, B(s) is unoccupied metal surface and AB is Adsorbed gaseous molecule.
1/n
Based on his theory, he derived Langmuir Equation which depicted a relationship between the number of active sites of the surface undergoing adsorption and pressure.

O = kp/1+ kp
Where  is the number of sites of the surface which are covered with gaseous molecules, P represents pressure and K is the equilibrium constant for distribution of adsorbate between the surface and the gas phase. The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only. At low pressure, kp is so small, that factor (1+kp) in denominator can almost be ignored so Lamgmuir equation reduce to  = kp

At high pressure, kp is so large, that factor (1+kp) in denominator is nearly equal to kp. So langmuir equation reduces to

 = kp/kp =1

3. BET Adsorption Isotherm:

BET Theory put forward by Brunauer, Emmett and Teller explained that multilayer formation is the true picture of physical adsorption. One of the basic assumptions of Langmuir adsorption isotherm was that adsorption is monolayer in nature. Langmuir adsorption equation is applicable under the conditions of low pressure. Under these conditions, lvii gaseous molecules would possess high thermal energy and high escape velocity. As a result of this, less number of gaseous molecules would be available near the surface of adsorbent (www. Xamplified. Com /adsorption,2010).

Under the condition of high pressure and low temperature, thermal energy of gaseous molecules would be available per unit surface area. Due to this, multilayer adsorption would occur. The multilayer formation was explained by BET Theory. The BET equation is given as

2.7.3.2 A NEW CLASSIFICATION OF ADSORPTION ISOTHERMS.

The first systematic attempt to interpret adsorption isotherms for gas-solid equilibra was introduced by Brunauer, Deming, Deming, and Teller (BDDT) in 1940 (Donohue,1995). These authors classified isotherms into five types. The BDDT classification has become the core of the modern IUPAC classification of adsorption isotherm, these BDDT isotherms and an P P0 1 - P P0 1 +C P P0 P P0 lviii additional one introduced much later by sing, which completes the IUPAC classification, are illustrated in figure 1 (Domohue,1995).

Type I is Southern characterize microporous adsorbents. Types II and III describe adsorption on macroporous adsorbents with strong and weak adsorbate – adsorbent interactions, respectively. Types IV and V represent adsorption isotherms with hysteresis. Finally, type VI has steps. The BDDT and IUPAC classifications have two deficiencies: they are incomplete and they give the incorrect impression that adsorption isotherms are always monotonic functions of pressure. This is because the IUPAC classification only takes into account adsorption at subcritical temperatures. A new classification of Adsorption Isotherms was analyzed by professor. Marc Donohue but will not be a subject of discussion here.

2.7.4 ACTIVATED CARBON

The most common industrial adsorbents are activated carbon, Silica gel, and alumina, because they present enormous surface areas per unit weigth. Activated/active carbon which is extremely porous is produced by roasting organic material to decompose it to granules of carbon-coconut shell, wood, and bone are common sources (wikipedia,2009). Although activated carbon is a magnificent material for adsorption, its black color persists and adds a grey tinge, even trace amounts are left after treatment, however filter materials with fine pores remove carbon quite well (Bungay,2000).

2.7.4.1 TYPES OF ACTIVATED CARBON

Activated carbons are complex products which are difficult to classify on the basis of their behaviour, surface characteristics and preparation methods.However, some broad classification is made for general purpose based on their physical characteristics.

a). Powdered activated carbon (PAC): These are activated carbon in form of powders or fine granules less than 1.0m in size with an average diameter between 0.15 and 0.25mm. Thus, they present a small diffusion distance.  PAC is made up of crushed or ground carbon particles, 95-100% of which will pass through a designated mesh sieve (wikipedia,2009).
b). Granular activated carbon (GAC): This is defined as the activated carbon being retained on a 50-mesh sieve (0.297mm). These carbons are preferred for all adsorption of gases and vapours as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodourisation and separation of components of flow system. GAC can be either in the granular form or extruded (wikipedia,2009).
c). Extruded activated carbon (EAC): Extruded activated carbon combines powdered activated carbon with a binder, which are fused together and extrude carbon block with diameters from 0.8 to 130mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content(wikipedia,2009).
d).Impregnated carbon:Porous carbon containing several types of inorganic impregnant such as iodine, silver, cations, such as Al, Mn, Zn, Fe, Li, Ca have also been prepared for specific application in air pollution control especially in museums and galleries. Due to antimicrobial/antiseptic properties, silver loaded activated carbon is used as an adsorbent for purification of domestic water. Drinking water can be obtained from natural lxi water by treating the natural water with a mixture of activated carbon and Al(0H)3 , a floccu -lating agent.
e). Polymer Coated Carbon: This is a process by which a porous carbon can be coated with a biocompatible polymer to give a smooth and permeable coat without blocking the pores. The resulting carbon is useful for hemoperfusion. Hemoperfusion is a treatment technique in which large volumes of the patients blood are passed over an adsorbent substance in order to remove toxic substances from the blood(wikipedia,2009).